A mainstream image forming method using paper as print media is currently an offset printing. The offset printing is a technique suited for mass printing. That is, a printing plate for an image is fabricated and set in a printing machine to make copies of the image at a rate of about 9,000 copies per minute. Disadvantages of the offset printing, such as time and cost required by a printing plate production process and a vast investment needed to purchase the printing machine, have little adverse effect on the cost per printed sheet and speed because the printed matter is produced in large quantities. It can therefore be said that the offset printing has matched market needs very well.
As a trend is gaining momentum in recent years for information versatility and more and more diversified printed matters are printed in small quantities, a problem has surfaced that the production cost of printing plates for individual printed matters becomes relatively high. Further, since instant availability of desired information is being given greater importance, there is a growing demand for a shorter production period in which a printed matter becomes available. Since the current offset printing has a long lead time, from a text preparation to a printing plate production and a printing familiarization (stabilizing of a printing machine), the production period cannot be shortened even when the number of printed copies is small. Further, since a huge facility investment is necessary and all processes require high levels of skills, production locations are limited, which means it takes time for printed matters to be delivered to customers.
In terms of meeting the market demands described above, the ink jet printing system is drawing attention as a desirable technology. Since the ink jet printing system uses no printing plate, it is suited for printing a small number of copies. Further, since it does not require large-scale facilities or a high level of specialty knowledge, desired printed matters can be produced on demand and therefore growing expectations are placed on the ink jet printing system.
Among points in which printed matters produced by the ink jet printing are inferior to offset-printed matters are a glossiness of printed matter, a printability on thin paper, a printability on both sides of paper and a printing cost. If improvements are made on these points, the ink jet printing system can be expected to advance toward commercial printing.
The glossiness of printed matter is affected largely by a surface smoothness of paper (print medium). The ink jet printing system has often used a penetration type ink that permeates paper and fixes in it. Since a colorant of ink fixes following the surface of paper, the paper used needs to have a highly smooth surface to produce a glossiness.
Paper with a highly smooth surface generally has a low ink absorbing capability. This is because a penetration type ink is absorbed through a capillary attraction. If printing is done on paper with a small ink absorption capability, ink may remain on the surface without being fully absorbed in paper, which may cause undesired phenomena, such as a bleeding in which the remaining ink becomes mixed with adjoining ink droplets and a beading in which previously landed ink droplets are drawn to subsequently landed ink droplets, resulting in a degradation of a printed image quality and a failure to dry properly. Under these circumstances, it is very difficult to form an image on paper with a high level of surface smoothness using the ink jet printing system without causing these problems.
The ink jet printing system is available in two types: a continuous type and an on-demand type, the latter type using electrothermal transducers (heating elements) and electromechanical transducers (piezoelectric elements). In either type only low-viscosity ink can be ejected. This is because the ink used in the ink jet printing system is required to be highly fluid while in the ink jet head to realize an adequate ink ejection performance. At the same time, on the surface of the print medium the ink is required to exhibit a low fluid characteristic to prevent adjoining ink droplets from getting mixed or from being drawn to each other. In the ink jet printing system as described above, while a highly fluid ink is ejected onto the print medium, the ink on the print medium needs to have a low fluidity. That is, opposing characteristics are required of the ink depending on whether it is in the print head or on the surface of the print medium.
To meet the contradictory requirements for the ink at the same time, a new system (an image forming system using an intermediate transfer body) is proposed in which an ink image is formed on a transfer body (or an intermediate transfer body), from which it is transferred onto a desired print medium to form the ink image on the print medium. In this system an ink ejected from the ink jet head is affixed to the transfer body temporarily to form an ink image on the transfer body whose fluidity is lowered to some extent while on the transfer body, and then the ink image is transferred from the transfer body onto the print medium.
When such a transfer body is used, it is desired that the surface of the transfer body be made a surface having a small ink absorbing capability or particularly a non-ink absorbing surface, considering an ink transferability from the transfer body to the print medium and an ease with which the transfer body can be cleaned after image transfer. However, if a transfer body with a non-ink absorbing surface is simply used, an ink on the transfer body remains fluidized, making it difficult to hold an ink image on the transfer body in good condition. That is, the use of the non-ink absorbing surface as the intermediate transfer body surface to enhance the transferability of an ink image from the intermediate transfer body makes it difficult to hold the ink image on the intermediate transfer body in good condition. Conversely, if the surface of the transfer body is made a surface that has a high ink absorbing capability to enhance the ability to hold the ink image on the intermediate transfer body, it becomes difficult to keep a good transferability of the ink image from the intermediate transfer body.
In the image forming system using the intermediate transfer body as described above, it is important in keeping a high quality of an ink image on the print medium to strike a good balance between a high level of capability to hold an ink image on the intermediate transfer body and a high level of transferability of the ink image from the intermediate transfer body. However, an image forming system has yet to be realized which establishes both a high level of capability to hold an ink image on the intermediate transfer body and a high level of transferability of the ink image from the intermediate transfer body and which can form a high quality of ink image on a variety of kinds of print media.
In Japanese Patent Application Laid-open No. 5-330035 (1993), for example, a method is proposed in which a transfer body is heated to increase a density of ink on the transfer body and thereby lower an ink fluidity on the transfer body. Simply heating the transfer body, however, can lower the ink fluidity only to a small extent, resulting in an ink image instantly spreading on the transfer body. That is, the ink image cannot be held in good condition on the transfer body, which in turn renders the ink image on a print medium after transfer unsatisfactory. This method has a problem that heat of the transfer body may reach an ink jet head and dry ink ejection nozzles, causing ejection failures. This method therefore has not yet been put to practical use.
Another method has been proposed which, as in Japanese Patent Application Laid-open No. 7-223312 (1995), uses a hot-melt ink and heats an ink jet head and an ink supply system to eject the melted hot-melt ink. In this case, however, since a thickness of affixed ink is large, an ink image formed on the print medium after transfer looks unnatural, making the quality of the image on the print medium after transfer less than satisfactory, similar to the method disclosed in Japanese Patent Application Laid-open No. 5-330035 (1993). When the hot-melt ink is used, the ink needs to be heated to a desired melted state. This melting process takes time and there are some limitations on the components of ink, leaving much to be desired.